What does a "double" refer to in trucking terminology?

In trucking terminology, a "double" specifically refers to a combination of two trailers being pulled by a single truck. This configuration allows for the transport of a larger volume of cargo compared to a standard single-trailer setup. The use of doubles is common in freight transport due to their efficiency and ability to maximize load capacity while still adhering to regulations regarding size and weight limits on the road.

The other options represent different concepts: a type of truck with two axles does not capture the essence of what a double entails, as it does not explicitly mention the trailer combination. A truck that can operate on dual fuel sources refers to the fuel system and is unrelated to the configuration of trailers. Lastly, a double-decker bus used in freight transport is a mischaracterization, as double-decker buses are primarily for passenger transport, not freight. Therefore, the term "double" accurately encapsulates the specific combination of a truck and two trailers in use for freight transportation.